1. Field of the Invention
The present invention relates generally to the control of an internal combustion engine having an air introduction device in the engine exhaust.
2. Background of the Invention
To reduce engine emissions, it is desirable to rapidly heat catalytic converters in the engine exhaust. In other words, catalytic converters achieve higher emission reduction after they have reached a predetermined operating temperature. Thus, to lower vehicle emissions, various methods are available to raise catalyst temperature as fast as possible.
In one approach, during the warm up phase of the catalytic converter, the engine ignition timing is retarded, and the engine air-fuel combustion mixture is set lean of stoichiometry. In this way, the retarded ignition timing provides additional heat to the engine exhaust, while the lean air-fuel mixture produces minimal hydrocarbons. In this way, catalyst temperature is rapidly increased, while minimizing hydrocarbon emissions. Such a system is cited in U.S. Pat. No. 5,497,745.
The inventors of the present invention have recognized that a disadvantage with the above approach is that the amount of ignition timing retard is limited by combustion stability. This in turn limits the maximum amount of heat that can be added to the catalytic converter, and thereby limits the ability to increase catalyst light off times.
Another approach to providing heat in the exhaust is to operate the engine with a rich air-fuel mixture while providing additional air directly into the engine exhaust. Such a system is described in U.S. Pat. No. 5,136,842.
The inventors herein have also recognized a problem with the above approach. In particular, the inventors have discovered that increased exhaust emissions may result due to incomplete burning of the excess hydrocarbons and injected air flow in the exhaust system. In other words, if the exhaust temperature is too low, complete burning in the exhaust manifold upstream of the catalytic converter may not occur. Also, the inventors herein have recognized that the engine fuel economy suffers while operating the engine rich and providing the additional air directly into the engine exhaust. In other words, the excess fuel added to provide a rich air-fuel ratio (and which provides heat via an exothermic reaction in the engine exhaust) detracts from vehicle fuel economy.
The above disadvantages with prior approaches are overcome by a method for operating an engine with an emission control device in an exhaust system of the engine, and an air introduction device coupled to the engine exhaust system. The method comprises; after an engine start, operating the engine with ignition timing retarded from optimal torque timing and combusting a lean air-fuel mixture; continuing said operation until an exhaust system temperature reaches a pre-selected value; and after reaching said pre-selected temperature value, operating the engine with a rich air-fuel mixture and commencing adding of air via said air introduction device.
By waiting until the exhaust temperature reaches a pre-selected value (for example a manifold auto-ignition temperature) it is possible to minimize the chance of unburned fuel passing through the exhaust system. Further, it is possible to use as little fuel as possible to achieve the desired rapid catalyst heating. In other words, the present invention withholds adding excess fuel to the engine until the exhaust temperature has reached a value that will support auto ignition of the rich exhaust gases and excess air introduced via the air introduction device.
Note that there are various methods for indicating when exhaust temperature reaches a pre-selected value. For example, an exhaust temperature sensor can be used which measures exhaust manifold temperature. Alternatively, the engine controller can estimate exhaust temperature based on various operating conditions such as, for example: intake air temperature, engine speed, ignition timing, and engine air-fuel ratio.
An advantage of the above aspect of the present invention is reduced emission due to decreased catalyst light-off times.